Electron gun having cathode with cylindrical extension and control grid with conical section

ABSTRACT

The rotationally-symmetric electron gun of a cathode-ray tube device comprises successively a cathode, a disk-shaped control grid provided with a central control, grid aperture and a diskshaped anode grid provided with a central circular-cylindrical anode grid aperture. The cathode comprises a first conductive circular cylinder from the base of which a second conductive circular cylinder projects the base of which forms the emitting surface. The control grid aperture, on the side of the anode grid, has a circular-cylindrical part and, on the side of the cathode, has a conical part which narrows towards the circularcylindrical part, the wall of which encloses substantially an angle of 45* with the axis of the electron gun. The diameter of the said first conductive circular cylinder and the diameter of the said circular-cylindrical part are substantially six times as large as the diameter of the emitting surface, while the diameter of the anode grid aperture is substantially four times as large, the length of the said second conductive cylinder is substantially twice as large, the distance between the emitting surface and the said circular-cylindrical part is substantially equally large, the axial length of the said conical part is at least equally large and the distance between the control grid and the anode grid is substantially one and a half times as large as the diameter of the emitting surface. The control grid is kept at a negative potential relative to the cathode. The anode is kept at a positive potential which is substantially four and a half times as large, relative to the cathode, while the space on the side of the anode grid remote from the control grid is kept substantially field-free.

United States Patent Van Roosmalen [451 Apr. 25, 1972 [54] ELECTRON GUN HAVINGCATHODE WITH CYLINDRICAL EXTENSION AND CONTROL GRID WITH CONICAL I SECTION 1 [72] lnventor: Johannes Hendrikus Roosmalen, Netherlands [73] Assignee: U.S. Philips Corporation, New York, NY.

[22] Filed: Nov. 12, 1970 [21] Appl. No.: 88,769

Theodorus Van Emmasingel, Eindhoven,

Primary Examiner-Robert Segal Arromey-Frank R. Trifari [57] ABSTRACT The rotationally-symmetric electron gun of a cathode-ray tube device comprises successively a cathode, a disk-shaped control grid provided with a central control, grid aperture and a disk-shaped anode grid provided with a central circular-cylindrical anode grid aperture. The cathode comprises a first conductive circular cylinder from the base of which a second conductive circular cylinder projects the base of which forms the emitting surface. The control grid aperture, on the side of the anode grid, has a circular-cylindrical part and, on the side of the cathode, has a conical part which narrows towards the circular-cylindrical part, the wall of which encloses substantially an angle of 45 with the axis of the electron gun. The diameter of the said first conductive circular cylinder and the diameter of the said circular-cylindrical part are substantially six times as large as the diameter of the emitting surface, while the diameter of the anode grid aperture is substantially four times as large, the length of the said second conductive cylinder is substantially twice as large, the distance between the emitting surface and the said circular-cylindrical part is substantially equally large, the axial length of the said conical part is at least equally large and the distance between the control grid and the anode grid is substantially one and a half times as large as the diameter of the emitting surface. The control grid is kept at a negative potential relative to the cathode. The anode is kept at a positive potential which is substantially four and a half times as large, relative to the cathode, while the space on the side of the anode grid remote from the control grid is kept substantially field-free.

3 Claims, 6 Drawing Figures Patented April 25, 1972 6 Sheets-Sheet 1 5/ /Vr////// p//V////// INVEN'I'OR.

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ROOSMALEN AGE NT ELECTRON GUN HAVING CATHODE WITH CYLINDRICAL EXTENSION AND CONTROL GRID WITH CONICAL SECTION The invention relates to a device comprising a cathode-ray tube, which cathode-ray tube comprises a rotationally-symmetric electron gun, in which electron gun are present successively a cathode provided with a plane electron-emitting surface extending substantially at right angles to the axis of the electron gun, a disk-shaped control grid provided with a central control grid aperture, and a disk-shaped anode grid provided with a central circular-cylindrical anode grid aperture, the control grid being kept at a negative potential relative to the cathode and the anode being kept at a higher positive potential relative to the cathode, the space on the side of the anode grid remote from the control grid being kept substantially fieldfree.

The invention also relates to a cathode-ray tube for such a device.

In the known devices of the type described, for example, devices having television camera tubes, television picture tubes, or oscillograph tubes, and electron microscopes, drawbacks present themselves as a result of lens defects, for example, the occurrence of an undesirably large spot and an undesirably large current inertia in a television camera tube. The electron beam obtains already in the electron gun a structure which deviates considerably from the ideal structure, since the lens formed by the cathode, the control grid and the anode grid causes so many spherical aberrations in the gun, that even electrons which leave the emitting surface of the cathode at the same initial speed intersect the axis of the gun at comparatively far divergent places.

Measures already proposed previously to restrict the spherical aberrations in the electron gun, for example, the individual or combined deformation of the anode grid with the intention of eliminating the lens defects thereof, the use of extra electrodes between the cathode and the anode grid, and the use of a spherically emitting surface, for example, a spherically emitting surface the radium of curvature of which varies as a function of the radial distance to the axis, so that the curvature decreases when the radial distance to the axis increases, are less efficacious than would be desirable and/or less simple to carry out than would be desirable.

It is the object of the invention to provide a device of the type mentioned in the first paragraph with which the abovementioned drawbacks are mitigated.

According to the invention, the cathode in a device of the type mentioned in the first paragraph comprises a first conductive circular cylinder from the base of which a second conductive circular cylinder projects the base of which forms the emitting surface and the control grid aperture, on the side of the anode grid, has a circular-cylindrical part and, on the side of the cathode, has a conical part which narrows towards the circular-cylindrical part, of which conical part the wall encloses substantially an angle of 45 with the axis of the electron gun, the diameter of the said first conductive circular cylinder and the diameter of the said circular-cylindrical part being substantially six times as large, the diameter of the anode grid aperture being substantially four times as large the length of the said second conductive cylinder being substantially twice as large, the distance between the emitting surface and the said circular-cylindrical parts and the axial length of the said circular-cylindrical part being substantially equally large, the axial length of the said conical part being at least substantially equally large and the distance between the control grid and the anode grid being substantially 1% times as large as the diameter of the emitting surface, the anode potential being substantially four and a half times as large as the control grid potential.

In this device of the said configuration and potentials of the electrodes of the electron gun, which configuration and potentials have been found after several calculations by again and again modifying the configuration and the potentials starting from a known configuration with optimum potential values to decrease the spherical aberrations, substantially no spherical aberrations are present any longer in the electron gun.

At high anode voltage, for example, at a voltage of the anode grid relative to the cathode of 300 volts, a particularly small cross-over, which is desirable notably in an electron microscope, is obtained owing to the small influence of the spreading as a result of thermal speeds of the electrons at said high potential.

At lower anode voltages, for example, at a voltage of the anode grid relative to the cathode of 10 volts, the electrons leaving the cathode with the same energy but at different angles with the axis of the electron gun, reach the cross-over while sharply separated. By means of a diaphragm arranged at the area of the cross-over and having a comparatively large diaphragm aperture which has radial dimensions, for example, of the order of magnitude of the diameter of the emitting surface, a particularly favorable selection of electrons having a small transverse speed can be obtained, while by means of this the electrons can also be selected properly for axial speeds, which can be very favorable, notably in a television camera tube, to obtain both a particularly small spot size and a particularly small beam current inertia. The invention therefore relates in particular to a device as described above, in which a diaphragm is present at the area of the beam cross-over.

The invention also relates to a cathode-ray tube for the device as described above. The cathode-ray tube comprises a rotationally-symmetric electron gun in which are present successively a cathode which is provided with a plane electronemitting surface extending substantially at right angles to the axis of the electron gun, a disk-shaped control grid provided with a central control grid aperture and a disk-shaped anode grid provided with a central circular-cylindrical anode grid aperture, and which is characterized in that the cathode comprises a first conductive circular cylinder from the base of which a second conductive circular cylinder projects the base of which forms the emitting surface and the control grid aperture comprises, on the side of the anode grid, a circular cylindrical part and, on the side of the cathode, has a conical part which narrows towards the circular-cylindrical part, of which conical part the wall encloses substantially an angle of 45 with the axis of the electron gun, the diameter of the said first conductive circular cylinder and the diameter of the said circular-cylindrical part being substantially six times as large, the diameter of the anode grid aperture being substantially four times as large, the length of the said second conductive cylinder being substantially twice as large, the distance between the emitting surface and the said circular-cylindrical part and the axial length of said circular-cylindrical part being substantially equally large, the axial length of the said conical part being at least substantially equally large and the distance between the control grid and the anode grid being substantially one and a half times as large as the diameter of the emitting surface.

In order that the invention may be readily carried into effect, it will now be described in greater detail, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of the electrode configuration of the electron gun in the cathode-ray tube of a known device, through the axis of the electron gun.

FIG. 2 shows the potential field and electron paths for electrons which leave the electron-emitting surface without initial speed in the known device shown in FIG. 1, in the half of the same cross-section,

FIG. 3 is a cross-sectional view of the electrode configuration of the electron gun in the cathode-ray tube of an embodiment of the device according to the invention through the axis of the electron gun.

FIG. 4 shows the potential field and the electron paths for electrons which leave the electron-emitting surface without initial speed in the device shown in FIG. 3, in the half of the same cross-section,

FIGS. 5 and 6 show the same electrode configuration and the same potential field as FIG. 4 with electron paths for electrons which leave the emitting surface with an initial speed in the plane of the drawing at different angles with the axis of the electron gun.

In the known electron gun configuration shown in FIG. 1, the cathode 1 has the emitting surface 2 which extends at right angles to the axis 3 of the electron gun. Opposite to the emitting surface 2 is arranged the disk-shaped control grid 4, in front of which the disk-shaped anode grid 5 is present which forms one assembly with the conductive anode cylinder which ensures that the space on the side remote from the control grid is kept field-free. The control grid has a circular-cylindrical aperture 7 and the anode grid has a circular-cylindrical aperture 8. The dimensions denoted by capitals in the figures have the following values:

A (the diameter of the control grid aperture 7) 600 ,u.. B (The diameter of the anode grid aperture 8) 400 LL. C (The distance between the emitting surface 2 and the control grid aperture 7) 100 u.

D (The axial length of the control grid aperture) 100 11,.

E (The distance between the control grid aperture 7 and the anode grid aperture 8) 150 p,

F (The axial length of the anode grid aperture 8) 200 [.L.

In this known gun, the voltages are, for example, chosen to be so that the negative control grid voltage V i.e. the voltage of the control grid relative to the cathode, is substantially threetenths of the positive anode voltage V i.e. the voltage of the anode grid relative to the cathode. When the ratio of the voltage of the electrodes varies, the lens strength varies strongly, but there is no voltage ratio at which the spherical aberrations are noticeably more unimportant than in the said voltage ratio.

FIG. 2 shows the electrostatic field in the electron gun shown in FIG. 1 for the following electrode voltages:

V 3 volts V volts.

The potential field is denoted by the equipotential lines, the potential values in volts being indicated in brackets. In the calculation, 9 is assumed to be the terminating equipotential plane, and linearly increasing potentials are assumed along 10 and 11. For illustration of the spherical aberrations, the figure shows electron paths which extend from the emitting surface 2 to the surface 9. These are electron paths for the electrons which start from the emitting surface at initial speed zero. As shown in the Figure, said paths intersect the axis of the gun at far divergent places. i

It is to be noted that in this figure, as in FIGS. 4, 5 and 6, the electron paths are reflected relative to the axis after having crossed the axis.

In the rotationally-symmetric electron gun configuration of an embodiment of the device according to the invention shown in FIG. 3, the cathode body consists of a conductive circular cylinder 12, from the base 13 of which the second conductive circular cylinder 14 projects the base of which forms the emitting surface 15. The control grid 16 has a control grid aperture which has a circular-cylindrical part 17 and a narrowing conical part 18. The wall 19 ofthe conical part 18 encloses an angle of 45 with the axis 20 of the gun. The diskshaped anode grid 21 forms one assembly with the anode cylinder 22, and has a circular-cylindrical aperture 23.

The dimensions have the following values:

G (The diameter ofthe emitting surface 15) i 100 p.

H (The diameter of the cylinder 12)= 6 G 600 [1,.

I (the diameter of the circular-cylindrical part 17 of the aperture in the control grid) 66 600 n. J (The diameter of the anode grid aperture 23) 4 G= 400 K (The length ofthe cylinder 14) 2G= 200 p.

L (The distance between the emitting surface 15 and the circular-cylindrical part 17 of the aperture in the control grid 16)= G= 100 .t.

M (The axial length of the circular-cylindrical part 17 of the aperture in the control grid 16) G 100 t.

N (The distance between the control grid 16 and the anode rid 21)=3/2 G= 150 y" 0 The axial length of the anode gridaperture 23) 200 ;1..

During operation an anode potential V is used which is substantially 4.5 X the negative control grid voltage V for example, an anode potential V 10 volts, and a control grid potential V 2.2 volts, the result illustrated in FIGS. 4, 5 and 6 being obtained.

As FIG. 2 shows the potential field and electron paths for electrons which start from the emitting surface at initial speed zero in the known electron gun shown in FIG. 1, FIG. 4 shows the same in the electron gun shown in FIG. 3. As shown in FIG. 4, the voltage V 2.2 volts, and V 10 volts. Reference numeral 24 denotes a terminating equipotential plane, and linearly increasing voltages are assumed along 25 and 26. The electron paths shown intersect in this case the axis 20 of the gun substantially at the same place.

FIGS. 5 and 6 show the same field as FIG. 4 while in FIGS. 5 and 6 electron paths are shown for electrons, which leave the emitting surface at an initial speed corresponding to 0.2 eV. at different angles a with the axis of the electron gun. In FIG. 5 it relates to paths for electrons which start at an initial speed having a component upwards in the figure. In FIG. 6 it relates to paths for electrons which start at an initial speed having a component downwards in the figure. As shown in the figures, paths are drawn for a =60, a 45, a=30, a 15 and a 0. For the same a, said electron paths intersect the plane 24 substantially at the same place. The Figures show separate target places for different values of a as a result of which a particularly favourable selection by means of a diaphragm is possible.

What is claimed is:

l. A device comprising a cathode-ray tube, which cathoderay tube has a rotationally-symmetric electron gun, in which electron gun are present successively a cathode provided with a plane electron-emitting surface extending substantially at right angles to the axis of the electron gun, a disk-shaped control grid provided with a central control grid aperture, and a disk-shaped anode-grid provided with a central circular-cylindrical anode grid aperture, the control grid being at a negative potential relative to the cathode and the anode being at a higher positive potential relative to the cathode, the space on the side of the anode grid remote from the control grid being substantially field-free, characterized in that the cathode comprises a first conductive circular cylinder from the base of which a second conductive circular cylinder projects the base of which forms the emitting surface and the control grid aperture, on the side of the anode grid, having a circular-cylindrical part and, on the side of the cathode, having a conical part which narrows towards the circular-cylindrical part, of which conical part the wall encloses substantially an angle of 45 with the axis of the electron gun, the diameter of the said first conductive circular cylinder and the diameter of the said circular-cylindrical part being substantially six times as large, the diameter of the anode grid aperture being substantially four times as large, the length of the said second conductive cylinder being substantially twice as large, the distance between the emitting surface and the said circular-cylindrical part and the axial length of the said circular-cylindrical part being substantially equally large, the axial length of the said conical part being at least substantially equally large and the distance between the control grid and the anode grid being substantially one and a half times as large as the diameter of the emitting surface, the anode potential being substantially 4% times as large as the control grid potential.

2. A device as claimed in claim 2, characterized in that a diaphragm is provided at the area of the cross-over of the electron beam produced in the cathode-ray tube.

3. A device as claimed in claim 2, characterized in that the diaphragm has an aperture the radial dimensions of which are of the order of magnitude of the diameter of the emitting surface.

UNITED STATES PATENT OFFICE 5/ 9 6 CERTIFICATE; OF CORRECTION Patent No. 3 ,659, 135 Dated April 25 L 1972 Inventor-9s) It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

. I '1 On the Title page, Section [30] change "691764" to Column 3, line 62, change 1001.1." to l00 u.--.

Signed and sealed this 25th day of July 1972.

(SEAL) Attest:

EDWARD MQFLETCHER,JR, I ROBERT GOI'TSCHALK Attesting Officer- Commissioner of Patents PC1-1050 UNITED STATES PATENT OFFICE r 4 (569 CERTIFICATE-OF CORRECTION Patent No. 3,659,135 Dated April 25, 1972 Inventorg/s) I It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below;

()n the Title page, Section [30] change 691764" to Column 3, line 62, change lOO u." to l0Q u.-.

Signed and sealed this 25th day of July 1972.

(SEAL) Attest:

EDWARD M.FLETCI-IER,JR. I ROBERT GUTTSCHALK Attesting Officer Commissionerof Patents 

1. A device comprising a cathode-ray tube, which cathode-ray tube has a rotationally-symmetric electron gun, in which electron gun are present successively a cathode provided with a plane electron-emitting surface extending substantially at right angles to the axis of the electron gun, a disk-shaped control grid provided with a central control grid aperture, and a disk-shaped anode-grid provided with a central circular-cylindrical anode grid aperture, the control grid being at a negative potential relative to the cathode and the anode being at a higher positive potential relative to the cathode, the space on the side of the anode grid remote from the control grid being substantially field-free, characterized in that the cathode comprises a first conductive circular cylinder from the base of which a second conductive circular cylinder projects the base of which forms the emitting surface and the control grid aperture, on the side of the anode grid, having a circular-cylindrical part and, on the side of the cathode, having a conical part which narrows towards the circular-cylindrical part, of which conical part the wall encloses substantially an angle of 45* with the axis of the electron gun, the diameter of the said first conductive circular cylinder and the diameter of the said circular-cylindrical part being substantially six times as large, the diameter of the anode grid aperture being substantially four times as large, the length of the said second conductive cylinder being substantially twice as large, the distance between the emitting surface and the said circular-cylindrical part and the axial length of the said circular-cylindrical part being substantially equally large, the axial length of the said conical part being at least substantially equally large and the distance between the control grid and the anode grid being substantially one and a half times as large as the diameter of the emitting surface, the anode potential being substantially 4 1/2 times as large as the control grid potential.
 2. A device as claimed in claim 2, characterized in that a diaphragm is provided at the area of the cross-over of the electron beam produced in the cathode-ray tube.
 3. A device as claimed in claim 2, characterized in that the diaphragm has an aperture the radial dimensions of which are of the order of magnitude of the diameter of the emitting surface. 